.spin-animation path {
  animation: custom 2s linear infinite;
}

@keyframes custom {
  0% {
    opacity: 0;
  }

  25% {
    opacity: 0.1;
  }

  50% {
    opacity: 0.2;
  }

  75% {
    opacity: 0.5;
  }

  100% {
    opacity: 1;
  }
}

.spin-animation path:nth-child(1) {
  animation-delay: 0s;
}

.spin-animation path:nth-child(2) {
  animation-delay: 0.5s;
}

.spin-animation path:nth-child(3) {
  animation-delay: 1s;
}

.spin-animation path:nth-child(4) {
  animation-delay: 2s;
}
